PLESIOSAURIA. B ( J7 



L. Lias (modified after C. W. Andrews), ag angular ; art articular ; b.occ basloccipial ; 

 bs basisphenoid ; d dentary ; ecpt ectopterygoid (transpalatine) ; fr frontal ; i.pt inter- 

 pterygoid vacuity ; / jugal ; mx maxilla ; nar external nares ; orb orbit ; pa parietal ; 

 pas parasphenoid ; pin pineal foramen ; pi palatine ; pmx premaxilla ; prf prefrontal ; pt 

 pterygoid ; pt.f postfrontal ; pt.nar internal nares ; pto postorbital ; qj quadratojugal ; 

 qu quadrate ; s supratemporal vacuity ; s.ag surangular ; so suborbital vacuity ; st supra- 

 temporal (prosquamosal) ; sq. squamosal ; v vomer. 



D Plesiosaurus dolichodirus : cervical vertebra, left side ; x J. L. Lias, r rib. 



E Cryptoclidus oxoniensis, transverse section of abdomen of immature specimen, x T \y 

 U. Jurassic, abd abdominal ribs ; r rib. 



F. Plesiosaurus dolichodirus ; caudal vertebra, anterior end, x J, L. Lias ; ch chevron 

 bone ; r rib. 



G 1 dorsal, 6 2 side view of pelvis of Muraenosaurus leedsi, x T V> Oxford Clay (after C. W. 

 Andrews), fe femur ; it ilium ; is ischium ; pb pub is. 



H pectoral, I pelvic limbs of the same specimen of Plesiosaurus dolichodirus, x -^, L. Lias 

 (Brit. Mus.). fe femur ; ft fibula ; h humerus ; i intermedium ; r radius ; t tibia; u ulna ; 

 x pisiform. 



J humerus or femur of Plesiosaurian in median longitudinal section, x j^, U. Jurassic. 

 c central cavity ; ep epiphyses ; s;shaft. (Brit. Mus.) 



orbital, to meet a large bone which overlaps the fixed quadrate and is 

 supposed to be equivalent to the supratemporal, squamosal and quadrato- 

 jugal. There is a pineal foramen. The internal nares are between the 

 vomers and the maxillae. The palatines are separated by the pterygoids 

 which touch for the greater part of their length, but are separated by an 

 interpterygoid vacuity behind. In the middle of the latter is the basi- 

 sphenoidal rostrum. In the triassic genera the pterygoids are in contact 

 throughout. The occipital condyle is single and mainly formed by the 

 basioccipital. Sclerotic plates have not been observed. The dentition 

 is thecodont and teeth are borne by the premaxillae, maxillae and man- 

 dible, and occasionally in the triassic genera by the pterygoids. 



In the triassic genera the pectoral girdle is on the ordinary reptilian 

 type, except that the coracoids meet in a ventral symphysis and there 

 is no sternum preserved. In the post-triassic forms however the pectoral 

 girdle presents some remarkable features. The symphysis of the coracoids 

 is much prolonged anteroposteriorly and the scapulae extend ventral- 

 wards towards each other, so as nearly or completely to meet in the median 

 ventral line in front of the coracoids. At the same time the clavicles and 

 interclavicle become reduced. 



The pelvic girdle is well developed (Fig. 220 G). The ilia articulate 

 with the ribs of the sacral vertebrae and all three bones enter into the 

 acetabulum, but the ilium joins the ischium only. There is a pubic and an 

 ischiadic symphysis. The limbs are more elongated and leg-like in the 

 triassic genera and have the normal number of phalanges, but in the 

 later forms they are more paddle-like and the bones of the fore-arm and 

 fore-leg shortened, and the phalanges are more numerous. 



Numerous close-set abdominal ribs are present, and the skin appears 

 to have been without scales and dermal plates. Vertical fins have not 

 been certainly made out. 



Fam. 1. Nothosauridae. Triassic Plesiosauria with elongated limbs 

 adapted for moving on land as well as in water. The palate is without 

 an interpterygoid vacuity. There are about 20 cervical vertebrae and the 

 cervical ribs are double-headed. The number of phalanges in both limbs 

 is normal. The coracoidal symphysis is not much extended longitudinally, 

 and the clavicles are well developed. Lariosaurus Curioni, Triassic shales 

 of Lake Como ; Pachypleura Cornalia (N eusticosaurus Seeley) U. Trias, 

 Lombardy etc. ; Dactylosaurus Giirich. Anarosaurus Dames, Cymato 

 saurus Dames, Muschelkalk, Silesia and Thuringia ; Pistosaurus, Simosau- 

 rus v. Meyer, Muschelkalk ; Nothosaurus Miinst., Muschelkalk, Germany 



